Engine braking strategies have long been utilized as a way of exploiting a rotating engine to slow a moving vehicle, such as an over the road truck. In the past, engine braking typically was accomplished by compressing air in a cylinder in a normal manner. But instead of injecting fuel to create power in the engine in the vicinity of top dead center, an exhaust valve or other valve is opened near top dead center to release the compressed air from the cylinder. Thus, by compressing and then releasing the compressed air in a blow down event, the engine does work and puts a retarding torque on a drive train coupled to the crank shaft. Engine braking has proven an effective way of slowing a moving machine without over reliance upon conventional wheel braking techniques. However, engine braking that involves opening a valve near top dead center in a compression stroke can produce excessive noise.
Partly in response to the noise problem associated with conventional engine braking, a new engine braking strategy was developed. For instance, European Patent EP0736672 shows an engine with a single intake and a single exhaust valve. This reference teaches engine braking by holding the exhaust valve slightly open during the compression stroke. By throttling flow across the exhaust valve seat, the engine does work and pressure builds within the cylinder but is evacuated past the throttled seat throughout the compression stroke. This work performed by the engine also creates a retarding torque, but does so relatively quietly with respect to conventional engine braking.
U.S. Pat. No. 7,013,867 teaches a similar braking strategy in an engine equipped with two exhaust valves joined by a valve bridge. In this reference, only one of the two exhaust valves is throttled to conduct engine braking. The other of the two exhaust valves is allowed to close during the compression stroke. When braking, a piston is hydraulically moved to a position that holds the one exhaust valve open slightly after the cam lobe passes. Thus, as both valves move toward respective closed positions one is held open by the brake actuator, and the other seats to remain closed for the braking event. If the non-braking valve seating velocity exceeds expected values, the valve and/or seat can prematurely wear out. Seating velocity of the unbraked valve can be relatively unrestrained due to the dynamics involved with actuating only one valve via the valve bridge while both valves are moving toward respective closed positions.
The present disclosure is directed toward one or more of the problems set forth above.